Heat-treating furnace



NOV- 25, l952 .1. WALLERIUS HEAT-TREATING FURNACE 3 Sheets-Sheet 2 FiledSept. 26. 1947 IN V EN TOR.

l3am Haga/w31 yan1 Hago/w31 NOV. 25, 1952 J WALLERHJS 2,619,341

HEAT-TREATING FURNACE Filed Sept. 26, 1947 3 Sheets-Sheet 3 I U l l IN VEN TOR.

John UQZZQTuS,

Patented Nov. 25, 1952 HEAT-TREATING FURNACE John Wallerius, Glen Ellyn,Ill., assignor to Sun- `beam Corporation, Chicago, Ill., a corporationof Illinois Application September 26, 1947, Serial No. 776,213

3 Claims.

The present invention relates to heat treating furnaces and moreparticularly to recirculating hot air or gas furnaces particularlyuseful for low temperature heating in many types of work. Such furnacesare very satisfactory for tempering carbon and alloy steels, forannealing brass between drawing operations, for stress relievingcastings, welding structures and cold worked metals, and for aging alltypes of castings. In addition almost all aluminum heating or heattreating and all heating operations and heat treating of magnesium canbe carried out in such furnaces.

The recirculating heat treating furnace has been extensively used inrecent years due to its eilicient and high speed operation. With theadvent of high temperature alloys it was possible to build fans whichwere trouble free and which could produce the large capacity forsatisfactory heating. It is a well-known fact that in a recirculatingfurnace perhaps the most important factor is fan capacity. It isnecessary to get high velocities of circulation to take full advantageof the convection principle for rapid uniform heating. vThe secondimportant requirement of welldesigned recirculating furnaces is an evendistribution of the flow of gases or air through the work. Thewidespread use of recirculating furnaces has resulted in the followingaccepted advantages; uniformity of temperature throughout the furnace,high speed heating at relatively low temperatures, high speed heatingthrough considerable depths of work, ability to maintain lowtemperatures without difficulty, operation through wide temperatureranges, capability of using oil as a fuel as well as gas or electricity,greater eniciency due to recirculation, and increased tonnage perfurnace.

In the recirculating furnaces now in extensive use it has been foundthat the work which is being heated at the air or gas entrance to theheating chamber initially attains a much higher temperature than thework at the air or gas exit and a substantial length of time is requireduntil the temperatures of the work at each end of the heating chamberare substantially the same. It has been discovered by extensive teststhat the fastest and most uniform heating is achieved when thetemperature drop of the gases through the work isa minimum. It isobvious that the temperature drop will be a minimum when the gases takethe shortest and most direct path through the work. The greater thenumber of particles of work with which each particle of air or gas comesin contact the coolerl the gas becomes and therefore the less heat canbe imparted to each succeeding particle of work. The greater the amountof turbulence, or stirring action, of the gases, the more the air or gasis cooled and the greater the variation of temperature of the workitself. The reduction of turbulence to a minimum has resulted inimprovement of all types of recirculating furnaces. Additional testshave shown that when a load of work was heated in one of these furnacesthe parts next to the inlet of the hot gases heated much faster thanthose near the outlet. This caused an ever increasing temperaturevariation through the work until a maximum point with respect totemperature variation was reached. Then, as the differential oftemperature between the incoming hot gases and the nearest parts becameless, more heat was left in the gases resulting in a relatively fasterheating rate for the parts near the outlet, This gradually reduced thetemperature variation through the load to an amount consideredsatisfactory for most work by the end of a reasonable heating period. Itwould be desirable to provide means for substantially reducing thetemperature variation with the consequent reduction in heating time andthe resulting increase in furnace capacity.

Accordingly it is an object of the present invention to provide a newand improved recirculating type of furnace with greatly reduced heatingtime for the same satisfactory service and with a substantial increasein furnace capacity.

It is another object of the present invention to provide a new andimproved recirculating furnace for heat treating purposes includingmeans for periodically reversing the direction of flow of gas or airthrough the work.

Still another object of the present invention is to provide a new andimproved recirculating furnace including numerous improved features fromthe standpoint of accessibility, speed of operation, simplicity, ease ofoperation and the like.

Further objects and advantages of the present invention will becomeapparent as the following description proceeds and the features ofnovelty which characterize the invention will be pointed out withparticularity in the claims annexed to and forming a, part of thisapplication.

For a better understanding of the present invention reference may be hadto the accompanying drawings in which:

Fig. 1 is a sectional elevational view of a furnace embodying thepresent invention, the sectional view lbeing taken substantially on lineI-I of Fig. 2, assuming Fig. 2 shows a complete structure;

Fig. 2 is a sectional view taken on line 2-2 of Fig. 1, assuming Fig. 1shows a complete structure;

the furnace taken in the direction of the arrows 3-3 of Fig. 2, with thefront wall removed to Fig. 3 is a side elevational view of a portion ofshow certain control and operating mechanism of the furnace;

Fig. 4 is a sectional view taken on line 4 4 of Fig. 1 assuming Fig. 1shows a complete construction;

Fig. 5 is a sectional View taken on line 5-5 of Fig. 2 assuming Fig. 2shows a complete structure;

Fig. 6 is a schematic diagram of the furnace of the present inventionillustrating the electrical control circuits therefor; and

Figs. 7 and 8 are curve diagrams to aid in understanding the operationof the recirculating furnace of the present invention.

Referring now to the drawings there is illustrated a recirculatingfurnace generally designated at I comprising a unitary structure formedof a frame of angle and channel members and provided with an exteriorwall generally designated at I I comprising parallel side walls I I aand IIb, a straight end wall llc, a curved end wall Ild, and a top wallIIe. The furnace I0 is specifically illustrated as a so-called batch orbasket type of furnace comprising a heating chamber generally designatedat I2 which is deiined by a suitable enclosing wall structure I3preferably formed of an alloy steel or the like to withstand the hightemperatures prevalent in the heating chamber I2. As illustrated thewall structure I3 is of circular configuration and is spaced from thecurved outer wall l Id of the furnacel by a substantial amount to dennea space which is filled with suitable heat insulating material generallydesignated at I4 which heat insulating material may comprise rock woolor any other suitable similar material, many varieties of which areavailable on the market today.

For the purpose of providing a furnace unit which is movable as aunitary structure a base generally designated at I5 defined by suitablechannel irons and the like is provided which supports the furnacestructure. The bottom of the heating chamber I2 is defined by asubstantial layer of an insulating cement generally designated at I6,which insulating cement provides a hard surface. This insulating cementbottom I6 is also supported by the base I5'.

For the purpose of providing a support for a work enclosing containerstructure there is provided at the bottom of the heating chamber I2 aring I8 preferably formed of an insulating concrete similar to thematerial I6 which is circumferentially disposed around the heatingchamber I2 so as to define a ledge I1. This ledge I1 has a discontinuousportion indicated at the lower left hand part of the heating chamber I2so as to define an air or gas passageway I9 which interconnects thebottom of the heating chamber I2 with the other parts of the furnace tobe described hereinafter.

In order that a suitable work containing structure may be centrallysupported in the heating chamber I2, the walls I3 of the heating chamberare provided near the ledge I1 with suitable guide members 20 arrangedin spaced relationship around the heating chamber just above the ledgeI1. 'Ihese guides 20 provide a centering arrangement so as to tend tocenter a work containing structure inserted into the heating chamber I2.

It will be understood that any suitable Work containing structure may beemployed for insertion and removal from the heating chamber I2. As amatter of fact if large pieces are to be heat treated they might beinserted directly into the heating chamber I2. However since the type offurnace illustrated is particularly adapted to heating batches of smallparts there is illustrated a work containing structure 22 in the form ofa cylindrical container preferably formed of an alloy steel or the liketo withstand the high temperatures in the heating chamber I2. The bottomof the cylindrical work containing structure 22 is designed to permitthe passageway of gas or air therethrough and is comprised of a gridstructure generally designated at 23 or any other grill work capable ofsupporting the work pieces in the container 22 and still permittingsubstantially unimpeded ow of gas or air in either direction. The workenclosing structure 22 may be open at the top or it may be provided witha suitable cover 24 having the necessary passageways therethrough forair or gas. If a cover 22 is employed it is generally employed for thepurpose of insuring the even distribution of such air or gas throughoutthe area defined by the cover 24. Also, in order that the workcontaining structure 22, with work for heat treating purposes, mayread'- ily be removed or inserted with reference to the heating chamberI2 it is provided with suitable handles 25 engageable by a crane orother suitable manipulating equipment. With this arrangement the basket22 may readily be loaded with work to be heat treated which of coursemay subsequently be unloaded to permit other batches of material to betreated. Preferably a. single heat treating furnace such as IU wouldemploy several work containing structures 22 so that the furnace may beemployed to its maximum capacity without delays due to loading orunloading new batches or permitting relativelyV slow cooling of treatedbatches.

It will be apparent that to permit insertion and removal of the workcontaining structure 22 from the heating chamber I2 that access to theheating chamber from the exterior must readily be afforded. In additionthe closure means provided for any opening to the heating chamber forthe removal and insertion of the work containing structure 22 must be aninsulating cover structure. As illustrated the heating chamber I2 isclosed by a suitable cover 28 which is of generally hollow wallconstruction so that the hollow space may be lled with a suitable heatinsulating material designated at 29 and preferably comprising rock woolor the like. For the purpose of providing a sealing surface around theopening to heating chamber I2 there is preferably provided a metal platesuch as 30. In order to make a proper seal between the cover 28 and themetal plate 30 the lower surface of the cover 28 is illustrated as beingprovided with four rings 3| arranged in spaced concentric relationship.These rings are fastened to the bottom of the cover as by welding or thelike and are arranged in pairs so as to define a relatively narrow spacebetween each pair of rings. This space is filled with a suitable hightemperature packing material generally designated at 32 so as to providetwo spaced concentric seals between the cover 28 and the plate 30surrounding the entrance to the heating chamber I2 in a manner which isclearly apparent from an examination of Fig. 1 of the drawings.

It will be apparent that the cover structure described above to providethe desired insulating features and the like has substantial mass. Inorder that this cover may readily be removed or replaced when access tothe heatingA chamber I2 is desired there is provided in accordance withthe present invention a suitable cover maparticularly when filled' 5,nipulating mechanism generally designated at 35 and best shown in Figs.3 and 4. This mechanism includes a suitable cover support 36 which isfastened in any suitable manner to the cover 28 and includes a portion36a which extends beyond the circumferential edge of the cover 28. Theportion 36a includes a vertically disposed opening for receiving avertically movable piston rod 38 which has its lower end connected to asuitable piston not shown, disposed within the cylinder 4I. To insurestraight line motion, the piston rod 38 passes through a pair of alignedsleeves 39 and 58 suitably and rigidly supported from a verticallydisposed channel member 42 positioned for also supporting the wall I Ib.The rod 38 is removably related to the support 36 as by the pin 43 andthe notch 44 best shown in Fig. 3.of the drawings. It will be apparentthat if air under pressure is supplied to the cylinder 4I below thepiston fastened to the piston rod 38, upward movement of the piston rod38 results with the consequent upward movement of the cover 28. Suchopening movement of the cover 28 still does not provide satisfactoryaccess to the opening to the heating chamber I2. In accordance with thepresent invention means are provided to produce subsequent pivotalmovement of the cover 28 about the axis of the connecting rod 38following initial upward movement thereof. As illustrated the sleeve 4I]which is provided with a suitable opening for the piston rod 38 so as topermit both axialA and rotational movement thereof relative thereto,includes a cam-like slot 46 for receiving a pin 41 fastened to thepiston rod 38. The slot 46 is so shaped that upward movement of thepiston rod 38 and consequently the pin 41 causes rotation of the shaftor piston rod 38 and consequently pivotal movement of the cover 28 awayfrom the opening to the heating chamber I2.

The remainder of the space defined by the walls II is divided intoseveral chambers including a control and mechanism chamber 58, a heatsource chamber I, a fan or blower generally designated at 52 and variousducts and mixing chambers to be described hereinafter. The fan or blower52 preferably comprises a suitable fan housing 53 and is illustrated inFig. 2 of the drawings, as substantially disposed on the longitudinalcentral axis of the furnace I8. Mounted for rotation in the fan housing53 is the fan or impeller 54 mounted on a suitable shaft 55 whichextends out of the fan housing 53 into the mechanism or control chamber58. As is best shown in Figs. 2 and 4 of the drawings, the fan housing53 is supported by a suitable support 56 from channel members 51 in turnsupported by the base I5. The fan chamber or fan housing 53 is providedwith an air or gas intake opening generally designated at 58 whichcauses the gas to flow into the housing in the direction of the axis ofthe shaft 55 so as to provide an axial inlet 58. On the other hand thefan has a circumferential outlet generally designated at 5S which outletis connected toducts 68 and 5I arranged in the form of a Y with thejuncture between the ducts 60 and 6I connected to the circumferentialoutlet 59. Each of the ducts 68 and 5I is connected by means of openings68a and Ia respectively with a return gas chamber 62 which is defined inthe space within the Y portion of the ducts 68 and 6I and the wall I3 ofthe heating chamber I2. The return gas chamber 62 as is best shown inFig. 2 of the drawings extends in the horizontal direction between the Yduct arrangement comprising the ducts 60 andIiI and a mixing chamber tobe described hereinafter. It will be understood that the fan parts areconstructed of suitable high temperature alloys to withstand the hightemperature air or gases to which they are subjected. It might bementioned at this time that in many heat treating operations air isemployed as the heating medium. However the present invention is equallyapplicable where some gas other than air is employed and it should beunderstood that the term gas as employed in this specication and claimsincludes air or any other gases which might be employed in the heattreating operation.

For the purpose of preventing substantial loss of heat from the fanhousing 53 or the air or gas ducts 58 or 6I, the space surrounding thesemembers is iilled with a suitable heat insulating material generallydesignated at 64 which heat rsulatlng material may be rock wool or theAlthough the source of heat employed in vthe furnace I0 of the presentinvention might comprise a gas burner, an oil burner, lor electricheater the invention has been specifically illustrated as comprising agas burner generally designated at 66 which is connected to a suitablesource of air and gas by a conduit means generally designated at 61.(See Fig. 6 of the drawings.) The gas burner is adapted to extendvintothe heat source chamber 5I which, as isA best shown in Fig. 4 of thedrawings, is dened by walls 68, 69 and 18 of an insulating nre brick andwall 1I of re brick not insulating in character. Preferably also a layerof ordinary insulating brick 12 is provided above the insulating firebrick layer 69. Upon operation of the gas burner 66 a gas flame isproduced in the heat source chamber 5I which causes hot gases to besupplied at the exit of the heat source chamber 5I. As is best shown inFig. 1 of thedrawings the wall 59 terminates in a curved portion 69a soas to direct the hot gases from the heat source chamber 5I in a downwarddirection into a mixing chamber generally designated at 14. This mixingchamber 14 is dened by the duct 15 which extends between the axial faninlet 58 and the return gas chamber 62, the term mixing chamber 'I4being appended thereto since it provides the chamber where the returngases from the chamber 62 are mixed with the hot gases from the heatsource chamber 5I which are guided into the mixing chamberv14 by thecurved wall portion 68a.

It will be understood that due to the burning of hot gases which areintroduced into the furnace, means must be provided for bleeding off anamount of gas equal to that continually being introduced by the gasburner 66, if one is employed. It will be understood that if the heatsource is electrical then this problem is not the same. In accordancewith the present invention a positive gas pressure exists in the returngas chamber 62 at the entrance to the mixing chamber 14 and at thispoint there is provided a conduit 19 connecting the return gas chamber62 to atmosphere outside the furnace whereby a predetermined amount ofreturned gas from the mixing chamber 14 may be vented to the atmosphereand no loss of hot newly burned gas results,

since the newly burned gases are supplied to the mixing chamber 14between the fan inlet 58 and the portion of the return gas chamber I62vented to atmosphere by the conduit 18. A

suitably adjustable damper 88 may be providedv 7 in the conduit 19 tocontrol the amount of gas venting, which may be substantially zero whenan electric heat source is employed.

To further insulate the working parts of the heating chamber therepreferably is provided a layer of asbestos` board 8| as is best shown inFig. 4 of the drawings between the insulating material described aboveand the exterior walls II of the furnace I0.

Thev mechanism and control chambei` 59 essentially comprises a spacedefined alongside the fan housing 53 between the channel member 42 andthe enclosing portions of the walls, IIb and IIc adjacent thereto.Disposed within this chamber is the cover-manipulating mechanism 35described above. Also a suitable motor 82.is suitably mounted withinthis chamber and drivingly connected with the fan shaft as by means ofcooperating pulleys 03 and 84 and a belt 85. Preferably a suitableopening 96 in the insulating wall structure is defined immediatelyadjacent the fan outlet 59 for the purpose of permitting connections tobe made with a suitable temperature measuring device designated at 81,and schematically indicated as a suitable pyrometer or the like in Fig.6 of the drawings.

As has been mentioned above if the heating chamber I2 is supplied withhot gases through operation of the fan 54, say for example in thedirection of the arrows shown in Fig. 1 of the drawings, the heating ofthe work at three different points therein is represented by the threecurves A, B, and C shown in Fig. 7 of the drawings Where the temperaturethereof is plotted with reference to time. The curve A represents thetop of the work which is first engaged by the hot circulating gasesindicating that it is clearly the hottest portion of the work untilafter a substantial time has elapsed. The curve C represents the pointof the work adjacent the hot gas exit which heats at a much slower rate.while the curve B represents the work in the middle of the container 22which heats at an intermediate rate. As sucient time elapses thetemperature differential between the top. bottom and middle of the workgradually disappears and substantially the same temperature isapproached. If the flow of gases were in the opposite direction curve Cwould remain the same and curves A and B would be interchanged.accordance wlth the present invention the gas circulation isperiodically reversed throughout the heating cycle to reduce to aminimum the dilerential in temperature between various parts of thework, and in Fig. 8 there are illustrated curves A', B', and C',corresponding respectively to the curves A, B and C, of Fig. '1, foranarrangement where periodic reversal of the air flow through the workoccurs. It is apparent from these curves that not only is thetemperature differential reduced but the time required to arrive at thesame end temperature is also greatly reduced.

For the purpose of providingV a simple means for reversing the flow ofair there is provided a damper 90 as shown in Fig. l of the drawingswhich is disposed at the junction point between the ducts and 6I. Thedamper 90 furthermore controls two paths of flow. When in the positionshown in Fig. 1 of the drawings the damper closes the opening 60avbetween the duct 60 and the return gas chamber 62 and also closes theopening between the fan outlet 5'9 and the duct 6| thereby requiring airor gasV to ow through the work fromV the top to the bottom.

If the damper 90 were rotated in a counterclock- Wise direction asviewed in Fig. 1 through an angle of substantially 90% the opening IiIawould be closed and the passageway between the fan outlet 59 and theduct 60 would also be closed so that the flow of air through the heatingchamber I2 would be from the bottom to the top. The now of air in allvother parts of the device would be unchangedr however, since the airwould 'flowfrom the return gas chamber 62 to the mixingy chamber 14 andfrom the heat source chamber 5I to the mixing chamber 14 in exactly thesame manner regardless of the'direction offlow through the heatingchamber I2. Obviously the flow from the mixing chamber 14 through theaxial fan inlet 58 is also unchanged regardless of the position of thedamper 90.

For the purpose of manipulating the damper 90 it is mounted on asuitable shaft 9| connected by a crank 92, a link 93, and a crank 94v toav unidirectional control motor 95. As illustrated in the drawings theunidirectional control motor 95 is preferably of the impulse typeextensively available on the market today which rotates only through anangle of upon the reception of an impulse and then rotates through asuccessivev 180 angle in the same direction upon the reception of adifferent impulse. Such a motor 95 generally is provided with fourterminals as indicated best in Fig. 6 of the drawings.

Any suitable control circuit for the furnace' Il may be provided. Asillustratedin Fig. 6 of ther drawings, the control circuit comprises a.source of potential |00, The pyrometer 81 is connected to a temperaturecontrol device IOI for controlling a valve |02 associated with the gasburner 66 thereby varying the temperature in the heat source chamber 5|,in accordance with the operation of the temperature control device IUIin a manner which will be understood by those skilled in the art.

For the purpose of providing the desiredperiodic reversal of gas owthrough the heating chamber I2 the damper motor 95 has two terminalsthereof connected directly by the conductors |93 and |04 with the powersource |00. The other two terminals of the motor 95 are connected byconductors |05 and |06, to one line of the power source |00 through thecontacts |01a and I01b respectively of a relay |01, the contacts I01abeing indicated as normally open contacts and the contacts |011) asnormally closed contacts. It will be apparent that with this arrangementthe conductors |95 and |06 are alternately connected to one conductor ofthe power source |00 in response to the alternate energiaation anddeenergization of the relay |01. The relay |01 forms a part of a timerunit generally designated by the reference numeral |08.

In order to provide a time control cycle, the control apparatus includesa timer motor generally designated at |09 including the winding |09a and|091) and the rotor |09c which is adapted to rotate in either a forwardor a reverse direction depending upon whether the winding |09a or thewinding I09b is energized. The rotor |09c includes a pair of spacedcontrol arms III and II2 for actuating the switches II3 and ||4respectively when the rotor |09c reaches its limiting positions. Thecontrol arm III is adapted to close normally open switch II3, in oneextreme portion of the rotor |09c while the arm I'I2 is adapted to opena normally closed switch ||4 when the rotor |09c assumes its otherextreme position. One terminal of each of the windings 9 |09a and |095of the motor |09 is connected directly to the same conductor |04 andthence to the power line |00. The other terminals of the windings |09aand |091; are connected through the contacts |`|c and |01d respectivelyof the relay |01 to the other terminal of the power line |00 through asuitable manual switch I 5 and the conductor |03. The winding |01 of therelay |07 is connected in parallel with the winding |0912 of a timermotor |09. With this arrangement when the switch ||5 is closed and theparts of the mechanism are in the position indicated in Fig. 6 of thedrawings, power is supplied to the Winding |09a of the motor |09 causingit to rotate in a direction to close the switch ||3. As soon as theswitch ||3 is closed, which occurs after the expiration of apredetermined time the relay |01 is energized to close its contacts |01awhich supply the motor 95 through the conductor |06 with the necessaryimpulse to produce 180 rotation of the control motor 95, with theconsequent movement of the damper 99 to reverse the flow of air to theheating chamber 22. Opening of the contacts |0`||d furthermoredeenergizes the winding |0902. Also closing of the contacts |0'lcenergizes the winding |09b to reverse the direction of the rotation ofthe rotor |090 of the timer motor |09. Subsequent opening of the switch||4 when the rotor |09c reaches its other extreme position causesdeenergization of the relay |01 and the winding |0919, with a further180 movement of the motor 95 and reversal of the position of the damper90. It will be apparent that with this arrangement alternate positionsof the damper 90 are assumed successively during a heating cycle independence upon any predetermined desired timing cycle.

In view of the detailed description included above the ope-ration of therecirculating furnace I0 will be apparent to those skilled in the artand no further discussion is included herewith.

Although the present invention has been described in connection with aso-called basket or batch type of recirculating furnace it should beapparent that it might equally well be employed with other types offurnaces such as the oven type of furnace, car or conveyor type, and thelike and the illustrated embodiment is by way of example only.

While there has been shown and described a particular embodiment of thepresent invention it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from theinvention in its broader aspects and it is therefore aimed in theappended claims to cover all such changes and modifications as fallWithin the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A recirculating furnace comprising a heating chamber adapted toreceive a removable container for the material to be heated, saidcontainer being provided with suitable openings in the ends thereof topermit the circulation of a gas therethrough, the combination of ablower disposed adjacent said heating chamber and having a gas inlet anda gas outlet, duct means for connecting said outlet with both ends ofsaid heating chamber, a source of hot gas, a mixing chamber disposedadjacent said inlet to said blower, means connecting said source to saidmixing chamber, a return gas chamber connected to said duct means, meansfor connecting said inlet to said mixing chamber, means connecting saidreturn gas chamber with said mixing chamber, means for venting toatmosphere gas from said return gas chamber, a single damper at saidoutlet for controlling the direction of flow of gas through said heatingchamber, and control means for periodically actuating said damper toreverse the flow of gas through said heating chamber.

2. In a recirculating furnace comprising a heating chamber adapted toreceive material to be heat treated, the combination of a blowerdisposed adjacent said heating chamber and having a gas inlet and a gasoutlet, duct means adapted to connect said outlet with both ends of saidheating chamber, a source of hot gas, a mixing chamber adjacent saidinlet to said blower, means connecting said source to said mixingchamber for injecting said hot gas into said furnace, a return gaschamber connected to said duct means, means for connecting said inlet tosaid mixing chamber, means connecting said return gas chamber with saidmixing chamber, means connected to said return gas chamber for bleedingoff an amount of gas comparable to that injected by said source, asingle damper at said outlet for controlling the direction of flow ofgas through said heating chamber, and control means for periodicallyactuating said damper to reverse the flow of gas through said heatingchamber.

3. In a recirculating furnace comprising in combination, a unitarystructure including a heating chamber adapted to receive material to beheat treated, a blower disposed adjacent said heating chamber and havingan axial inlet and a gas outlet, duct means adapted to connect saidoutlet with both ends of said heating chamber, a hot gas chamber, amixing chamber disposed adjacent said axial inlet, means connecting saidhot gas chamber to said mixing chamber for supplying newly heated gas tosaid mixing chamber, a return gas chamber connected to said duct means,means connecting said axial inlet to said mixing chamber, meansconnecting said return gas chamber with said mixing chamber, meansconnected to said return gas chamber for venting a predetermined amountof gas to atmosphere, an adjustable damper in said last mentioned means,a single pivotal damper disposed at said blower outlet for controllingthe direction of flow of gas through said heating chamber, and controlmeans for periodically actuating said damper to reverse the flow of gasthrough said heating chamber.

JOHN WALLERIUS.

REFERENCES CITED The following references are of record in the file ofthis patent:

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